This invention pertains to novel cellulosic sizing agents and to an improved process for sizing cellulosics.
Cellulosics, such as paper, are often sized with various materials to impart water-resistance and resistance to aqueous solutions and dispersions, e.g., inks. This "water-resistance" is necessary in many instances to retain the strength of the cellulosic and to facilitate processing operations, such as printing, etc.
The term "cellulosic," as used herein, is meant to include paper, paperboard and other fibrous sheet-like or molded masses derived from wood, wood pulp, cotton, or other source of cellulose fibers. The term "cellulosic" also includes sheet-like or molded masses prepared from combinations of cellulosic and noncellulosic materials (e.g., polyamides, polyesters, polyacrylic acid resin fibers, and mineral fibers such as asbestos, glass, etc.). Thus, the term includes textiles, such as cotton-polyester blends.
Previous cellulosic sizing agents have generally been hydrophobic materials, such as rosin, fortified rosin, mixtures of rosin with waxes, etc. Their utility in most instances was limited by one or more factors, e.g., solubility, stability, adhesion to the cellulosic, pH dependence, etc. Their pH dependence was one of the more important limiting variables, particularly in sizing paper, since most were effective and operable only under acid conditions. A sizing agent for paper which may be used at neutral or alkaline pH values is very desirable since paper prepared under such conditions has greater strength, resistance to aging, etc., than paper prepared under acidic conditions, and, alkaline pigments such as calcium carbonate and alum can be used in the aqueous pulp slurry without adversely affecting the sizing agents or sized material.
In some instances, aziridinyl-based compounds have been used as cellulosic sizing agents.
One such sizing agent was described by G. H. Brown and M. M. Skoultchi in U.S. Pat. No. 3,575,796. They used an aqueous emulsion of certain monomeric N-substituted aziridines in sizing paper and paperboard products. Their aziridines were prepared by reacting ethylenimine (aziridine) with an .alpha.,.beta.-ethylenically unsaturated compound, such as distearyl maleate, stearyl acrylate, etc. The sizing agents were typically dispersed in aqueous media with cationic starch, which functioned both as retention aid and a cationic emulsifier.
Other aziridinyl-based paper sizing agents are described in "Ethylenimine and Other Aziridines" by O. C. Dermer and G. E. Ham, Academic Press, N.Y. (1969), pp. 362-3.
In other instances, starch and starch derivatives have been used to impart sizing to cellulosics. For example, U.S. Pat. No. 3,017,294 illustrates the use of a cationic starch as a paper size.